The present invention relates to a life-saving or rescue system for a marine structure, and it is particularly intended for use in connection with offshore structures such as drilling rigs, production plattforms and the like.
Conventional rescue systems used on ships are based on life boats supported in davits by means of which they are swung free of the ship's side and lowered into the water. Davits are rather complicated apparatus requiring frequent inspection and testing to keep the risk of malfunction at a reasonably low level. The launching operation, including boarding, swinging and lowering the life boat, requires a relatively long time, and in cases of rough sea there is always the risk of the life boat being crushed against the ship side during lowering or after release.
Recently a simpler and safer rescue system has been developed in which the life boat, which is of the enclosed type, from a horizontal storing and boarding position on board the ship, is allowed to slide down a ramp and fall freely therefrom to plunge into the water bow foremost and with its longitudinal axis extending in a preferred angle of impact. The drop energy of the boat will then be converted into positive propulsion of the boat in a direction away from the ship.
When using the above described rescue system the angle of impact of the life boat relative to the water surface, in addition to the height of the drop, will determine the acceleration loads at the moment of impact, as well as the subsequent propulsion. If the angle of impact is too steep the boat will rebound, and if the angle is too small the shock forces on hull and passengers will be excessive for drops above a certain limit. Therefore, if the rescue system is to function properly, the angle of impact when launching the life boat must be within specified limits. The angle of impact in turn depends on the launching angle, i.e. the degree of incline of the ramp initiating the boat's motion at the time of launching. When leaving the ramp upon launching a rotary torque will be applied to the boat resulting in a constant fore- and -aft rotation of the freely falling boat. Consequently it will be necessary in each case to calculate the adequate ramp incline for different heights of fall, and the uncertainty factor associated with the angle of impact increases with the height through which the boat will be falling. Any list and/or trim of the structure carrying the life boat will further influence the angle of launch and consequently the angle of impact when launching a life boat according to the above method. Use of this system is presently approved only for drops up to about 20 meters, which means that its application is mainly limited to ships and semisubmersibles at operational draft.
However, also offshore structures such as production platforms etc., are in demand of a life saving or rescue system having a simpler construction and quicker operation than conventional implements now in use. In this case, however, the drop height may be excessive so that the above mentioned system cannot be used due to the uncertainty as to the angle of impact. Thus, an object of the present invention is to provide a rescue system based on an enclosed life boat constructed to perform a free fall when launched, in which the impact angle of the boat with the water surface is substantially independent of its height of fall and of any list of the marine structure carrying it.